1. Field of the Invention
The present invention relates to a process for producing a triazine derivative useful as a herbicide and a process for producing an intermediate compound for the production of the triazine derivative.
2. Description of the Prior Arts
As a triazine-type herbicide, WO90/09378 discloses a triazine derivative in which a phenoxyalkylamino group is substituted on a triazine ring, as is shown in the following formula. This publication describes that the above triazine derivative has remarkable advantages that it not only has excellent herbicidal effect but also exhibits no phytotoxicity on paddy rice. ##STR4## wherein R.sup.1 is a C.sub.1 -C.sub.4 alkyl group, n is an integer of 0 to 2, R.sup.2 is hydrogen or C.sub.1 -C.sub.2 alkyl group, and X is a halogen atom.
The above publication discloses a process for producing the above triazine derivative, which comprises reacting a biguanide derivative such as 2-phenoxy-1-methylethyl biguanide with a halocarboxylic acid ester such as 2-fluoroisobutyric acid ester in the presence of a base. This process is specifically illustrated by the following reaction scheme. ##STR5##
However, the process for producing a triazine derivative, described in the above WO90/09378, has the following defect; The halocarboxylic acid ester as one of the starting materials is hydrolyzed due to water formed as a byproduct in the reaction, and the reaction between the biguanide derivative and the halocarboxylic acid ester therefore cannot be quantitatively carried out. Hence, the yield of the intended triazine derivative is low. This defect can be overcome to some extent by using an excess amount of the halocarboxylic acid ester relative to the biguanide derivative as a starting material. In this case, however, the amount of the halocarboxylic acid ester increases, and the production cost consequently increases.
Meanwhile, 2-fluoroisobutyric acid ester, one of halocarboxylic acids used for the production of the above triazine derivative, is obtained, for example, by carrying out a halogen exchange reaction in which AgF is allowed to act on 2-bromoisobutyric acid ester for halogen exchange in the absence of a solvent (J. Org. Chem. 33 4279 (1968)). This process is illustrated by the following reaction scheme. ##STR6##
In the above conventional process, methacrylic acid ester is formed as a byproduct at a step of producing 2-fluoroisobutyric acid ester from 2-bromoisobutyric acid ester. Therefore, the defect with this process is that the selectivity to the intended substance, 2-fluoroisobutyric acid ester, is as low as 20%. Further, there are also another defect in that AgF as a reagent for the halogen exchange reaction is expensive and the production cost inevitably increases.
Further, J. Org. Chem. 55 3423 (1990) discloses a process for producing 2-fluoroisobutyric acid ester, in which trimethylsilyl chloride is allowed to react with methyl isobutyrate in the presence of lithium diisopropylamine (LDA) to prepare 1-methoxy-1-(trimethylsilyloxy)-2-methylpropene, and then F.sub.2 is allowed to react therewith in CFCl.sub.3 to obtain 2-fluoroisobutyric acid ester. This process is illustrated by the following reaction scheme. ##STR7##
The defect with the above conventional process using methyl isobutyrate as a raw material is that it uses a two-step reaction and the yield of the intended product decreases. There is also another defect that the reactions in the first and second steps are required to be carried out at an extremely low temperature of -78.degree. C. and therefore special equipment such as a refrigerator is required.
Further, as shown in the following reaction schemes, there are also an HF addition reaction of methacrylic acid ester and a substitution reaction using HF in which a hydroxy group of hydroxyisobutyric acid ester is replaced with fluorine. ##STR8##
In the above processes, however, HF is a highly toxic reagent and difficult to handle, and further, the reactions frequently entail a side reaction. It is therefore difficult to control these reactions.